Flatband voltage shift of ruthenium gated stacks and its link with the formation of a thin ruthenium oxide layer at the ruthenium/dielectric interface

A systematic study about the flatband voltage (V_fb) shift of Ru gated metal-oxide-semiconductor stacks after thermal treatment in O₂ has been performed. The dependence of the V_fb shift on the anneal time and temperature and the thickness of Ru was studied in detail, and a clear link between the V_fb shift and an oxygen diffusion process in Ru was observed. A high temperature thermal treatment of the devices prior to the O₂ anneal has no significant impact on the V_fb shift. The V_fb shift is ascribed to the shift of metal gates’ work function, and is not intrinsic to HfO₂ gated stacks as similar behavior was also observed on SiO₂, from the combination of internal photoemission and conventional capacitance-voltage measurement. No similar V_fb shift was observed for TiN gated stacks and the V_fb shift seems to be more related to the properties of gate electrodes other than those of gate dielectrics. After thermal treatment in ¹⁸O₂, from time-of-flight secondary ion mass spectrometry measurement, it was found that ¹⁸O penetrated through Ru and was incorporated at around the Ru/dielectric interface, which corresponded to the formation of an interfacial RuO_x layer. The thin interfacial RuO_x layer was found to have very similar properties as the ones of bulk RuO₂ and the mechanism of its formation was discussed- from thermodynamics and kinetics points of view. We believe that the formation of a thin RuO_x layer at around the Ru/dielectric interface in O₂ ambient is responsible for the increase of the V_fb for Ru gated stacks.